Method and apparatus for increasing the useful life of a quartz envelope in a high power light source

ABSTRACT

In a high power light radiator having a light transmitting quartz shell with a total radiation loading incident upon such shell in the order of at least about 2000 watts per sq. inch of surface area with a proportional part of such radiation having wavelengths below 1600 A wherein the quartz susceptibility to deterioration is decreased by the addition of an additive such as titanium or an oxide thereof to said quartz.

United States Patent 1191 Troue METHOD AND APPARATUS FOR INCREASING THE USEFUL LIFE OF A QUARTZ ENVELOPE IN A HIGH POWER Nov. 26, 1974 3,581,139 5/1971 Haftetal. ..313/221 3,597,650 8/1971 Anderson m1. ..313/231 LIGHT SOURCE Primary Examiner-Ronald L. Wibert [75] Inventor: Harden Henry Troue, Indianapolis, Assistant ExammeF'Rwhard Rosenbergef 1nd Attorney, Agent, or Firm-Eugene Lieberstem [73] Assignee: Union Carbide Corporation, New

57 ABSTRACT [22] Filed: Nov. 27, 1972 In a h1gh power light radiator having a light transmlt- PP 309,808 ting quartz shell with a total radiation loading incident upon such shell in the order of at least about 2000 52 us. c1 313/221, 313/112, 313/231 watts p qinch of surface area with a Proportional [51 1m. 01. HOlj 5/04 P of such radiation having wavelengths below 1600 [58] n w of Search 313/221, 231, 184 7 A wherein the quartz susceptibility to deterioration is decreased by the addition of an additive such as tita- 5 References Cited nium or an oxide thereof to said quartz.

UNITED STATES PATENTS 4 Claims, 3 Drawing Figures 3,53l,677 9/l970 Loughndge 3l3/22l POWER SUPPLY PATENTL 591251974 3. 851 302 mm 1 0f 2 24 POWER SUPPLY FIG.- IA

PAIENTE; Bv 261914 3.851.202 mm 2 a; 2

WAVELENGTH (A) (yool 13- wn/sum) ALISNlLNI 3 UY'IOSSV 'IVWHON FIGZ METHOD AND APPARATUS FOR INCREASING THE USEFUL LIFE OF A QUARTZ ENVELOPE IN A HIGH POWER LIGHT SOURCE This invention relates generally to high power arc light sources and more particularly to an apparatus and method for increasing the useful-life of the light transmittingenvelope in a high power arc light source and in particular to a light source whose arc generates a high intensity predominately continuous spectral distribution of light including ultraviolet short wave radiation below 1600 Angstroms.

The use of high power arc light radiators, particularly for generating radiant energy in the short wave length ultraviolet region, is becoming especially important in the field of irradiating photo chemicals. Present day devices for generating high intensity light are of the arc radiation type wherein a pair of electrodes are spaced at opposite ends within an arc chamber defined by a transparent tubular envelope which is almost invariably formed from quartz. These devices usually operate with a continuous flow of swirling inert gas injected into the pressurized chamber in the space between the electrodes. Although any inert gas may be employed, argon is preferred because of availability and economics of operation.

In industrial applications for which such are light sources are intended it is essential that such sources be capable of operating reliably for relatively long accumulated times, 100 hours at the very minimum. Heretofore, the transparent window or envelope experienced failure, often in the form of an explosion, after very short accumulated operating times, from only a few to at most tens of hours. The failures would occur suddenly, frequently without any apparent indication of envelope deterioration. To overcome this serious deficiency double concentric envelopes were used. The inner envelope was used to define the arc chamber while the outer envelope, radially spaced from the inner envelope, took the greater portion of the pressure loading and in some instances was used to maintain a compressive force upon the inner envelope. A cooling gas was introduced in the space between the envelopes to cool and reduce the thermal gradient over the inner envelope. Although the use of two envelopes did increase the useful life of the arc light source, the increase in operating time before failure was not ade quate. Moreover, double envelopes had some serious disadvantages, namely; they increased the complexity and cost of the sources and they decreased the radiant output since there were two mediums through which the light had to be transmitted.

The present invention is concerned solely with increasing the reliability and permissible accumulated operating times of arc light sources by increasing the longevity of the quartz envelope, i.e., rendering the envelope less susceptible to deterioration and more tolerable to the harsh environment and intense radiation to which it is subjected.

To prolong quartz life primary attention had heretofore been given to the theory that defects in quartz purity as well as optical defects act as a starting point for deterioration. Based upon this reasoning all initial attempts to improve longevity involved selection of the Very best of quartz materials in terms of their purity and UV range of transmission such as (by trade name) Amerisil, Superasil, Dynasil UV-l000, Thermal American Spectrosil, and Quartz Silice Pursil 453. Such materials at failure exhibited few visible signs of deterioration but had accumulated lifetimes of only a few tens of hours of operation before failure. Moreover, failure often occurred in the form of an explosion.

It was subsequently discovered, contrary to what would have been anticipated from the aforementioned concept of quartz failure, that the presence of titanium or any oxide thereof in the quartz, as an additive to the extent of at least about 3 parts per million, increases the life expectancy of the quartz by at least an order of magnitude from tens to hundreds of hours. It is theorized from such discovery that any additive which has an absorption band below 1600 A. and including 1049 A will beneficially affect quartz longevity. One immediate and obvious advantage of the above noted discovery is that the use of a second or outer envelope is not necessary. It has further been observed that, with the inclusion of titanium or any oxide thereof in the quartz, or presumably any additive that meets the above noted requirements, any deterioration which occurs does so slowly and is visibly detectable,providing ample warning and time to replace the envelope thereby substantially reducing the possibility of an explosive failure.

These and other advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 schemetically illustrates, in longitudinal vertical section, a typical high power are radiation source to which the invention is uniquely adapted;

FIG. 1A illustrates a section of one embodiment of the quartz envelop of FIG. 1 where the quartz is composed of laminated sections;

FIG. 2 is a graph of a typical spectral radiance distribution of the light emitted from the radiation source of FIG. 1.

The illustrated apparatus shown in FIG. 1 generally comprises a pair of spaced electrodes 12 and 14 respectively, which are axially aligned within a pressurized gas tight arc chamber 16 formed from a transparent quartz envelope 20. Each electrode is of nozzle type construction having a centrally disposed bore. It is to be understood however that the invention is neither limited to the specific arc source assembly shown in FIG. 1 nor to hollow type electrodes. Argon gas is injected into the arc chamber 16 from gas inlet port. 22 in a swirling pattern so as to create a strong swirling gas flow within the chamber. The gas exits from the bores of each electrode. An arc 18 is established between the electrodes and is maintained by power supply 24. The are 18 terminates at each electrode within the nozzle bore at a location relatively close to the adjacent ends of the electrodes. The swirling gas constricts the arc column 18 increasing the arc intensity and resulting in are current densities of the order of 2000 amp/cm or greater.

The quartz envelope 20 forming the pressurized arc chamber 16 must not only efficiently transmit the radiant energy emitted from the arc column 18 with as little loss as possible, but must be strong enough to withstand the pressure within chamber 16 and the heat radiated by the very intense arc. The are column 18 emits a continuum radiation spectrum having a typical distribution as shown in FIG. 2. The total radiant energy striking the envelope is at least about 2000 watts per square inch of quartz surface area. Quartz will transmit radiation in accordance with the following equation:

I T e-04M. wherei V I transmitted radiation,

1,, incident radiation,

04 wavelength dependent absorption coefficient,

t quartz thickness,

A wavelength.

The magnitude of a is inversely related to wavelength in the short ultraviolet region. It is well known that quartz efficiently transmits radiation above about 1800 A where (1 is relatively small; whereas below l800 A, in the short wavelength ultraviolet region, the coefficient of absorption becomes very large. Since the arc column 18 emits very little radiation below 1800 A, the intense radiation of the arc column incident upon the quartz envelop 20 should play an insignificant factor in the rapid deterioration of the quartz. Although some radiation at less than 1800 A will will be emitted from argon atoms which are swept outwardly from the vicinity of the arc column 18 to a relatively cooler region, such radiation cannot be a signifcant factor primarily because of its comparitively low intensity. Moreover much of such radiant flux will strike or cross the arc column 18 where it would be readily reabsorbed. For the above reasons primary attention has, in the past, been given to the physical and thermal loading on the quartz envelope.

After a considerable reevaluation it was concluded that in fact resonance radiation from argon at about 1049 A was initiating stress concentrations deep within the quartz envelope walls. What earlier had been analyzed as thermal strain concentrations within the walls were determined to be the result of structural damage from the absorption of argon resonance radiation at about 1049 A. Resonance radiation results when an electron which has been brought to an excited state returns from its lowest excited state to its unexcited energy level or ground state emitting a quantum of radiant energy. The predominant resonance line for argon is 1049 A. This wavelength is readily absorbed throughout the wall structure of the quartz envelope thereby stressing and structurely weakening it.

Although it would be expected that radiation corresponding to this resonance line would be reabsorbed within the arc column and should generally never leave the arc, it was felt that the escape of such radiation must in fact be taking place. Moreover, such rapid stress concentration and quartz structural deterioration as observed required a relatively intense level of such radiation.

It was ultimately discovered that the source of such intense 1049 Angstrom resonance radiation-is the arc termination regions and/or tail flame within the bore of each nozzle electrode where sufficient cooling exists to retum the electrons to the ground state and release resonance radiation. Hence, it was concluded that the stress concentration and structural deterioration of the quartz envelope walls could be prevented or at least significantly reduced if the penetration of the quartz wall by such radiation could be significantly decreased.

It was demonstrated pursuant to the present invention that if an additive, such as titanium or any oxide thereof, which has a very large absorption coefficient, below 1600 A, is present within the quartz, a heavy absorption band below 1600 A and including 1049 A results which significantly decreases the quartz susceptibility to penetration by short wavelength ultraviolet radiation below 1600 A. Although titanium and/or its oxides is preferred any additive having this characteristic absorption band may be employed. For purposes of the present disclosure oxides of titanium specifically include TiO and TiO but is not limited thereto. Quartz is sold commercially based on its degree of purity. Some poorer grades of commercially available quartz have been chemically analyzed and are known to contain a great many impurities including trace quantities of titanium and/or its oxides. It was found that the level of concentration of titanium and/or its oxides has to be at least about 3 parts per million to secure a significant increase in quartz envelope longevity when the quartz envelope is subjected to intense radiation from an arc radiation source where a proportional part of such radiation is below 1600A. The use of a fused quartz material containing at least about 5 parts per million titanium and/or its oxides extended the useful life of the quartz envelope tenfold when compared to the useful life achieved under otherwise identical operating conditions using an envelope of essentially pure quartz material having only a trace amount of titanium and/or its oxides.

Further analysis indicates that the distribution and location of the additive within the quartz envelope wall volume is also significant. It is preferred that the additive be unifomtly concentrated primarily at the inner surface of the quartz envelope wall as opposed to being distributed throughout the quartz envelope wall volume. This will further limit the penetration of the destructive short wave ultraviolet 1049 A radiation and permit ready visible detection of the extent of envelope deterioration. A preferred technique to impart a greater concentration of titanium and/or its oxides at the inner surface of the quartz envelope wall would be to fabricate a laminated quartz envelope with the inner layer thereof containing the largest concentration of the additive.

What is claimed is:

1. A method for increasing the useful life of a quartz medium for use as a light transmitting member in a plasma arc light source having a swirling gas flow of substantially argon passed thereabout and wherein said quartz medium is in the form of a tubular shell surrounding said arc and defining the arc chamber comprising adding an additive to said quartz in an amount of at least 3 parts per million and selected from the group consisting of titanium and an oxide of titanium.

2. A method for increasing the useful life of a quartz medium for use as the light transmitting envelope in an arc radiation plasma type light source wherein a proportional part of the radiated light incident upon said member is short wave ultraviolet radiation of below 1600 Angstroms comprising the steps of fabricating a laminated quartz envelope having at least one lamina including at least about 3 parts per million of an additive selected from the group consisting of titanium and an oxide of titanium.

3. In an apparatus for generating high intensity light radiation having a continuum spectral distribution extending from the ultraviolet region to at least the visible region, including in combination; a pair of spaced electrodes between which an arc is established, a single tuing of titanium and an oxide of titanium whereby said envelope is rendered less susceptible to deterioration from short wavelength ultraviolet energy.

4. In an apparatus as defined in claim 3 wherein a greater proportion of said additive is contained within said envelope at the inner surface thereof facing said are. 

1. A method for increasing the useful life of a quartz medium for use as a light transmitting member in a plasma arc light source having a swirling gas flow of substantially argon passed thereabout and wherein said quartz medium is in the form of a tubular shell surrounding said arc and defining the arc chamber comprising adding an additive to said quartz in an amount of at least 3 parts per million and selected from the group consisting of titanium and an oxide of titanium.
 2. A method for increasing the useful life of a quartz medium for use as the light transmitting envelope in an arc radiation plasma type light source wherein a proportional part of the radiated light incident upon said member is short wave ultraviolet radiation of below 1600 Angstroms comprising the steps of fabricating a laminated quartz envelope having at least one lamina including at least about 3 parts per million of an additive selected from the group consisting of titanium and an oxide of titanium.
 3. In an apparatus for generating high intensity light radiation having a continuum spectral distribution extending from the ultraviolet region to at least the visible region, including in combination; a pair of spaced electrodes between which an arc is established, a single tubular quartz envelope surrounding said electrodes and defining therewithin an arc chamber, said envelope representing the sole light transmitting enclosure, means for passing argon gas in a swirling pattern within said arc chamber and power supply means for maintaining said arc, the improvement of which comprises; said single quartz envelope including at least 3 parts per million of an additive selected from the group consisting of titanium and an oxide of titanium whereby said envelope is rendered less susceptible to deterioration from short wavelength ultraviolet energy.
 4. In an apparatus as defined in claim 3 wherein a greater proportion of said additive is contained within said envelope at the inner surface thereof facing said arc. 